Protective face coverings, including lens-containing structures such as masks and goggles that cover the eyes while still providing appropriate visual acuity, are used in a number of applications. These include eye protection in manufacturing or industrial circumstances, laboratories, educational activities, home and garden tasks, and in medical applications such as surgery.
With respect to medical applications, the eyes provide one of the main available entry points into the human body for viruses, microbes and other organisms. Thus, in medical practices such as surgery, where exposure to blood and other bodily fluids of a patient are expected, eye protection represents more than protection against loss of vision, and specifically provides a barrier against entry of potentially serious or even fatal diseases. For example, the increase in Acquired Immune Deficiency Syndrome (AIDS) in the population as a whole, and the present lack of a known cure, has increased the risk that infected patients or their blood will transmit the virus to exposed medical personnel through their eyes.
From a medical perspective, one desirable characteristic of an appropriate face mask, goggle, or other lens is that it be easily used in a sanitary condition. The simplest method of maintaining a goggle or a mask in a sanitary condition is to manufacture it under sterile conditions, use it once and then dispose of it. Accordingly, desirable masks and goggles should be manufactured from materials for which the cost is low enough to make disposal practical. One such material is polyester. When properly manufactured using known techniques and equipment, polyester is transparent, rigid enough to form an appropriate shape, optically clear, and flexible enough to be formed into light weight, comfortable masks, goggles and similar lens structures. Furthermore, as evidenced by its wide use in disposal food packaging, polyester is a relatively low cost material. It can also be conveniently recycled. Thus, goggles or masks formed from polyester can provide appropriate protection at low enough cost to be considered disposable.
One problem that arises with lens-containing devices, however, is the problem of fogging. In general, fogging occurs on a surface when the temperature of that surface falls below the dew point of the surrounding ambient air. With respect to lenses, fogging tends to occur because of the proximity of the surface of the lens portion of a goggle or a mask to the head and eyes of the wearer. As the wearer perspires or warms the air between the eyes and the lens (and even if not visibly so), moisture present between the lens and the wearer has a tendency to condense on the interior surface of the lens. If the lens material is hydrophobic, the condensed moisture will fail to wet the surface, and instead will form small droplets. Observed cumulatively, these droplets diffract light sufficiently to interfere with or even block the vision of the wearer. Thus, fogging is most desirably avoided where delicate work requires high visual acuity, such as during medical procedures, some of which are carried out at relatively low temperatures for medical reasons. For example, some surgical procedures are carried out at ambient temperatures as low as 50.degree. F. in order to help slow a patient's metabolism.
Polyester, although favorable in a number of qualities discussed above, is nevertheless one such hydrophobic material and is thus subject to fogging. Accordingly, in order for polyester to be useful as a lens material in medical and other such applications, some technique must be used to minimize or eliminate fogging.
A typical method of attempting to avoid or eliminate fogging is to add some sort of hydrophilic coating to a hydrophobic substrate. When moisture condenses on a hydrophilic surface, it tends to wet the surface rather than form drops. As a result, the wetted surface is often transparent enough to prevent vision through the lens from being impaired.
In other techniques hydrophobic antifogging coatings have been applied to lenses in an attempt to cause water vapor to avoid contact entirely with the lens. Unfortunately, such techniques tend to have a net effect of instead allowing large water droplets to form, which is as undesirable as the fogging.
Even the hydrophilic coatings, however, include certain problems. Typically many exhibit poor optical clarity, cracking, streaking or haziness, opalescence, adhere poorly to substrates, have an oily surface, and are difficult to apply in a uniform coat.
A number of other techniques include the use of silicon compounds as antifogging agents, but such techniques bring their own set of problems are relatively expensive for a disposable article. Other compounds are insoluble in appropriate coating solvents and thus raise application difficulties.
One potentially favorable hydrophilic coating is polyvinyl alcohol (PVOH). As known to those familiar with this compound, polyvinyl alcohol is produced by the polymerization of vinyl acetate, followed by hydrolysis to form alcohol functional groups.
Polyvinyl alcohol, however, generally will not adhere to common lens substrate materials such as polyester, polycarbonate, acrylic, or cellulosic films. This lack of adhesion has limited the use of polyvinyl alcohol and the advantages it otherwise offers.
Accordingly, many modifications have been attempted in order to increase the affinity of polyvinyl alcohol for a substrate such as polyester. For example, some techniques add other functional groups to the backbone of the polymer in an effort to increase the affinity to the substrate. As the chemical nature of the polyvinyl alcohol is modified, however, the hydrophilic nature is also modified and its effectiveness as an antifogging material is minimized or destroyed.
Other techniques attempt to add wetting or surface active agents to the polyvinyl alcohol, but these materials are only effective when they bloom to the surface of the polyester and in doing so exhibit an oily condition which in turn creates optical problems. Typically a lens material containing such surfactants creates a mottled appearance which is generally unacceptable.
Other methods of preparing the substrate have similarly failed to provide desired results, with such techniques including corona discharge treatment of the substrate, or of the coatings and primers. Such techniques have generally lacked success, however, usually because of poor adhesion to the substrate, poor optical properties or both.